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991.
该文介绍了一种建立树木生物量模型的简单快速方法——嵌套式回归。基本原理是以枝轴为基本单位, 逐级拟合。过程是把枝条分解成枝轴, 从枝轴到枝条, 再到单株, 拟合不同层次或尺度的生物量模型。建立枝轴生物量方程, 估计各级枝轴生物量, 将枝轴生物量(实测值或模拟值)总和起来便得到枝条生物量。由于样本单元之间有包含关系, 实际测定的样本很小, 具有快速实用的特点。检验结果显示, 模型预测值和实测值具有较高的一致性。 相似文献
992.
993.
Hubbell’s neutral theory claims that ecological patterns such as species abundance distributions can be explained by a stochastic
model based on simple assumptions. One of these assumptions, the point mutation assumption, states that every individual has the same probability to speciate. Etienne et al. have argued that other assumptions on the speciation process could be
more realistic, for example, that every species has the same probability to speciate (Etienne, et al. in Oikos 116:241–258, 2007). They introduced a number of neutral community models with a different speciation process, and conjectured formulas for
their stationary species abundance distribution. Here we study a generalised neutral community model, encompassing these modified
models, and derive its stationary distribution, thus proving the conjectured formulas. 相似文献
994.
Lowland savannas are a rare variant of Midwestern United States savanna occurring on alluvial soils, for which reference information is sparse. To evaluate the appropriateness of using upland savanna as a surrogate source of reference information for lowland savanna, we studied a pre‐Euro‐American lowland savanna using original U.S. Public Land Survey data and other historical records. Historical vegetation was reconstructed and compared among upland savannas, lowland savannas, and lowland forests; we also evaluated potential disturbance dynamics maintaining these systems. We found that all three communities were dominated by members of the genus Quercus but also had extensive representation by many other tree species, especially notable for savannas in this region. There were no clear size–density relationships for species in the genus Quercus, indicating that these historical savannas were not characterized exclusively by large, scattered oak trees but rather by trees of many oak species and nonoak species in a wide range of size classes. Both upland and lowland savannas also contained a substantial shrub component. We found no evidence that lowland savannas were maintained by flooding, although the uneven‐aged canopy structure suggested that periodic disturbance occurred. Restoration of lowland savanna in this region should include provisions for maintaining nonoak species and shrubs, with disturbance timed to maintain an uneven‐aged canopy structure. Although the appropriateness of historical data in the face of climate change may be questionable, in this region, a warmer climate may actually help promote the “oak parkland” that was present from 8,000 BP up to Euro‐American settlement. 相似文献
995.
Question: Static sampling designs for collecting spatial data efficiently are being readily utilized by ecologists, however, most ecological systems involve a multivariate spatial process that evolves dynamically over time. Efficient monitoring of such spatio‐temporal systems can be achieved by modeling the dynamic system and reducing the uncertainty associated with the effect of design choice at future observation times. However, can we combine traditional techniques with dynamic methods to find optimal dynamic sampling designs for monitoring the succession of a herbaceous community? Location: Lower Hamburg Bend Conservation Area, Missouri, USA (40°34′42″ lat. 95°45′38″ long.). Methods: The dynamic nature of the system under study is modeled in such a way that uncertainty in the measurements and temporal process can both be accounted for. Both fixed and roving monitoring locations were used in conjunction with a spatio‐temporal statistical model to efficiently determine optimal locations of roving monitors over time based on the reduction of uncertainty in predictions. Results: During the first 3 years of the study, roving monitors where held at fixed locations to allow for statistical parameter estimation from which to make predictions. Optimal monitoring locations for the remaining 2 years were selected based on the overall reduction in prediction uncertainty. Conclusions: The dynamic and adaptive vegetation monitoring scheme allowed for the efficient collection of data that will be utilized for many future ecological studies. By optimally placing an additional set of monitoring locations, we were able to utilize information about the system dynamics when informing the data collection process. 相似文献
996.
PATRICK J. MULHOLLAND BRIAN J. ROBERTS WALTER R. HILL‡ JOHN G. SMITH 《Global Change Biology》2009,15(7):1767-1776
Some expected changes in climate resulting from human greenhouse gas emissions are clear and well documented, but others may be harder to predict because they involve extreme weather events or heretofore unusual combinations of weather patterns. One recent example of unusual weather that may become more frequent with climate change occurred in early spring 2007 when a large Arctic air mass moved into the eastern United States following a very warm late winter. In this paper, we document effects of this freeze event on Walker Branch, a well‐studied stream ecosystem in eastern Tennessee. The 2007 spring freeze killed newly grown leaf tissues in the forest canopy, dramatically increasing the amount of light reaching the stream. Light levels at the stream surface were sustained at levels considerably above those normal for the late spring and summer months due to the incomplete recovery of canopy leaf area. Increased light levels caused a cascade of ecological effects in the stream beginning with considerably higher (two–three times) rates of gross primary production (GPP) during the late spring and summer months when normally low light levels severely limit stream GPP. Higher rates of stream GPP in turn resulted in higher rates of nitrate (NO3?) uptake by the autotrophic community and lower NO3? concentrations in stream water. Higher rates of stream GPP in summer also resulted in higher growth rates of a dominant herbivore, the snail Elimia clavaeformis. Typically, during summer months net NO3? uptake and snail growth rates are zero to negative; however, in 2007 uptake and growth were maintained at moderate levels. These results show how changes in forest vegetation phenology can have dramatic effects on stream productivity at multiple trophic levels and on nutrient cycling as a result of tight coupling of forest and stream ecosystems. Thus, climate change‐induced changes in canopy structure and phenology may lead to large effects on stream ecosystems in the future. 相似文献
997.
This study examines the impact of the exotic nitrogen-fixing legume Melilotus officinalis (L.) Lam. on native and exotic species cover in two Great Plains ecosystems in Badlands National Park, South Dakota. Melilotus is still widely planted and its effects on native ecosystems are not well studied. Melilotus could have direct effects on native plants, such as through competition or facilitation. Alternatively, Melilotus may have indirect effects on natives, e.g., by favoring exotic species which in turn have a negative effect on native species.
This study examined these interactions across a 4-year period in two contrasting vegetation types: Badlands sparse vegetation
and western wheatgrass (Pascopyrum smithii) mixed-grass prairie. Structural equation models were used to analyze the pathways through which Melilotus, native species, and other exotic species interact over a series of 2-year time steps. Melilotus can affect native and exotic species both in the current year and in the years after its death (a lag effect). A lag effect
is possible because the death of a Melilotus plant can leave an open, potentially nitrogen-enriched site on the landscape. The results showed that the relationship between
Melilotus and native and exotic species varied depending on the habitat and the year. In Badlands sparse vegetation, there was a consistent,
strong, and positive relationship between Melilotus cover and native and exotic species cover suggesting that Melilotus is acting as a nurse plant and facilitating the growth of other species. In contrast, in western wheatgrass prairie, Melilotus was acting as a weak competitor and had no consistent effect on other species. In both habitats, there was little evidence
for a direct lag effect of Melilotus on other species. Together, these results suggest both facilitative and competitive roles for Melilotus, depending on the vegetation type it invades. 相似文献
998.
J. Stephen Athens 《Biological invasions》2009,11(7):1489-1501
Paleoenvironmental and archaeological investigations from the ’Ewa Plain of O’ahu provide insight into the problem of understanding
lowland native forest loss in Hawai’i. Data from pollen analysis of a pond core record, avian paleontology, and archeology,
document a precipitous decline of the native forest starting before Polynesian settlement on the ’Ewa Plain but after Polynesian
colonization of O’ahu. It is hypothesized that rats, introduced by Polynesian colonizers, increased exponentially in the absence
of significant predators or competitors, feeding on a largely endemic vegetation that had evolved in the absence of mammalian
predators. Rats radiated ahead of human colonizers on O’ahu, eating their way through the vegetation, perhaps before the colonizers
had encountered much of the pristine lowland forest into which the rats had radiated. This hypothesis is supported by several
observations, including the almost complete absence of extinct or extirpated avian faunal remains in archaeological deposits,
the present distribution of endemic vegetation in Hawai’i, rat ecology, population biology, and other evidence.
相似文献
J. Stephen AthensEmail: |
999.
Environmental and Biotic Controls over Aboveground Biomass Throughout a Tropical Rain Forest 总被引:1,自引:0,他引:1
Gregory P. Asner R. Flint Hughes Timothy A. Varga David E. Knapp Ty Kennedy-Bowdoin 《Ecosystems》2009,12(2):261-278
The environmental and biotic factors affecting spatial variation in canopy three-dimensional (3-D) structure and aboveground
tree biomass (AGB) are poorly understood in tropical rain forests. We combined field measurements and airborne light detection
and ranging (lidar) to quantify 3-D structure and AGB across a 5,016 ha rain forest reserve on the northeastern flank of Mauna
Kea volcano, Hawaii Island. We compared AGB among native stands dominated by Metrosideros polymorpha found along a 600–1800 m elevation/climate gradient, and on three substrate-age classes of 5, 20, and 65 kyr. We also analyzed
how alien tree invasion, canopy species dominance and topographic relief influence AGB levels. Canopy vertical profiles derived
from lidar measurements were strong predictors (r
2 = 0.78) of AGB across sites and species. Mean AGB ranged from 48 to 363 Mg ha−1 in native forest stands. Increasing elevation corresponded to a 53–84% decrease in AGB levels, depending upon substrate age.
Holding climate constant, changes in substrate age from 5 to 65 kyr corresponded to a 23–53% decline in biomass. Invasion
by Psidium cattleianum and Ficus
rubiginosa trees resulted in a 19–38% decrease in AGB, with these carbon losses mediated by substrate age. In contrast, the spread of
former plantation tree species Fraxinus uhdei corresponded to a 7- to 10-fold increase in biomass. The effects of topographic relief at both local and regional scales
were evident in the AGB maps, with poorly drained terrain harboring 76% lower biomass than forests on well-drained relief.
Our results quantify the absolute and relative importance of environmental factors controlling spatial variation in tree biomass
across a rain forest landscape, and highlight the rapid changes in carbon storage incurred following biological invasion.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.
Author Contributions GPA and RFH conceived of or designed the study. GPA, RFH, TAV, DEK, and TKB performed research and analyzed data. GPA, RFH,
DEK, and TKB contributed new methods or models. GPA wrote the article. 相似文献
1000.